The present invention relates to a reinforced connection part of chemical connectors such as an anisotropic electroconductive membrane, elastomer connector, etc. and a method of construction therefor. It concerns, in particular, improvements in reliability in high-temperature regions during long term use.
Recently, chemical connectors such as anisotropic electroconductive membranes, elastomer connectors, etc. have been used for the connection of high-density finepattern electrodes such as a Liquid Crystal Display (LCD), a Light Emitting Diode (LED), an Electroluminescence (EL), etc. to a flexible printed circuit (FPC). For example, when the transparent electrode (hereinafter abbreviated as ITO membrane electrode-ITO) being an electrode on the side of an LCD, and a FPC to drive it are joined, a method of heating under pressure allowing the anisotropic electroconductive membrane (membrane conducting to the direction of thickness but not conducting to the direction of width) to lie between both has been adopted for adhesion. But, since the base resin of this electroconductive membrane is of hot-melt type, the adhesive force tended to decrease at a raised temperature. Moreover, in the case of the so called zebra connector, a sheet-like connector made in such a way that silicone type rubber and electroconductive layer admixed fine powders such as carbon black etc. with said rubber are superposed alternately is pressed to be joined by external force. However, with simple pressing by said external force, there arise inconveniences such as the occurrence of poor junctions etc. because of the creep etc. of materials occuring at the time of long-term use. In order to improve on this, a method of joining both electroconductive portions is carried out, wherein a heat seal connector, the electroconductive portion consisting of electroconductive pastes such as carbon, Ag, etc. being formed in the base film of adhesive layer by a printing method, is allowed to lie between said ITO and FPC and brought into adhesion under heat.
Recently, with the advent of the coloration of LCDs and the requirement for more distinction of image quality, it has become necessary to make the clearance between conductors in the ITO portion provided on the screen of an LCD even smaller. Specifically, the pitch between electrodes to be provided for an ITO membrane electrode has dropped from 0.4 mm to a level of 0.3 or 0.2 mm, and the heat seal connector used in the printing method aforementioned has not been able to correspond to this any longer. In addition, since the use fields of LCDs have increased from so-called disposable consumption type products such as watches, camera and TVs to long-life, high-reliability and durability type products, for example, terminal input-output devices of work station and OA instruments and planar display products for cars, for which high functionality is required, the connector system used up to this time as described above has become unsatisfactory.
In such a situation, a method is known wherein not only said high-density fine-pattern electrode and FPC are pressed simply through a chemical connector, but also the connection part is fastened tightly with suitable metal fittings, for example spring materials for the reinforcement. By this method, however, sufficient tightening cannot be obtained, since the size of the connection part, for example the thickness, is extremely small, i.e., around 1.5 mm, the size of a spring is restricted so that the deformation quantity of the spring cannot be large, and the superelasticity of the spring is also a strain of less than 1% at most. Moreover, with a large spring, which occupies a large space, the miniaturization and the thinning of instruments are difficult, and with a small spring the attachment thereof to the connection part by deforming it within a limit of elastic deformation is difficult. Furthermore, the spring materials have such properties that the tightening force varies significantly depending on the extent of strain, and there is a shortcoming that the tightening is lowered drastically if creep deformation, etc. are caused in the constituent materials of the connection part, for example.
For this reason, in Japanese Unexamined Patent Publication No. Sho 61-203698, such a shape memory alloy is disclosed as a metal fitting to hold down and reinforce the connection part. By using this, many advantages can be obtained such that the reinforcement of the connection part of an electronic circuit becomes extremely easy, the discrepancy of position between that at the time of attachment and that at fixation does not occur compared with the method to press down physically, the force of pressing can be controlled by the thickness and the shape of the metal fitting and, at the same time, there are no partial floatings and gaps occuring when using spring materials, so that uniform contact force can be obtained, and the like. However, the reinforcing metal fitting described in the specification of the foregoing patent has a shape as shown in FIG. 2 (G) and, because of the long legs thereof, the contact with the connection part of electronic circuit becomes a surface contact, resulting in that the nonuniformity of the holding area is inevitable. Moreover, in the reinforcing metal fitting, superelasticity is necessary, but there is no concrete description about this fact in the specification thereof.
As a result of diligent investigations with respect to these points, the inventors have discovered that the shape and the superelasticity of reinforcing metal fitting (hereinafter referred to as a fastener) comprising the shape memory alloy affect significantly the reliably at high-temperatures in use over a long term, leading to the present invention.